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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        // 得到1到100相加的和 5050   (1 + 100)* 100 /2
        var a = 1;
        var sum = 0;
        while ( a<=100 ){
           sum += a;
           a ++ ;
        }
        alert("1-100的数相加的和是" + sum);
    </script>
</body>
</html>